Wireless communication systems which can dynamically adapt to constantly changing environmental propagation characteristics will be the key for the next generation of communication applications.
The antenna is an extremely important component in any wireless appliance because it transmits and receives radio waves. An antenna operates as a matching device from a transmission line to free space and vice versa. An ideal antenna radiates the entire power incident from the transmission line feeding the antenna from one or more predetermined direction. Performance of the antenna dictates performance of most wireless devices and hence is a critical part of the system.
Antenna configuration determines the antenna properties that include impedance and VSWR (Voltage Standing Wave Ratio), amplitude radiation patterns, 3 dB beamwidth, directivity, gain, polarization and bandwidth. Different antenna configurations have different antenna properties.
A reconfigurable antenna is one which alters its radiation, polarization and frequency characteristics by changing its physical structure. The reconfigurable antenna concept is fundamentally different from a smart antenna.
A smart or adaptive antenna is an antenna array of elements that are typically standard monopoles, dipoles or patches. A signal processor is used to manipulate the time domain signals from or to the individual antenna elements by weighting and combining elements of the signals to change the resulting radiation pattern, i.e. the spatial response of the array, satisfies some conditions. This is the key concept of beam forming through which electromagnetic energy is focused in the direction of the desired signal while a null is placed in the direction of noise or interference sources.
Patch Antennas consists of a metallic patch over a dielectric substrate that sits on a ground plane. The antenna is fed by a microstrip line or a coaxial cable line. A microstrip patch antenna is a resonant style radiator which has one of its dimensions approximately λg/2 where λg is the guided wavelength.
The patch acts as a resonant cavity with an electric field perpendicular to the patch that is along its z direction. The magnetic cavity has vanishing tangential components at the four edges of the patch. The structure radiates from the fringing fields that are exposed above the substrate at the edges of the patch. A microstrip antenna can be fabricated in many shapes, for example, square, circular, elliptical, triangular, or annular.
Microstrip Patch Antennas have several well-know advantages over the other antenna structures, including their low profile and hence conformal nature, light weight, low cost of production, robust nature, and compatibility with microwave monolithic integrated circuits (MMIC) and optoelectronic integrated circuits (OEIC) technologies.
Micro-electro Mechanical Systems (MEMS) switches are devices that use mechanical movement to achieve a short circuit or an open circuit in the RF transmissions line. RF MEMS switches are specific micromechanical switches that are designed to operate at RF-to-millimeter-wave frequencies (0.1 to 100 GHz) and form the basic building blocks in the RF communication system. The forces required for the mechanical movement can be obtained, for example, but not exclusively using electrostatic, magneto static, piezoelectric, or thermal designs.
The advantages of MEMS switches over p-i-n-diode or FET switches are:                Near-Zero Power Consumption: Electrostatic actuation does not consume any current, leading to very low power dissipation (10-100 nJ per switching cycle).        Very High Isolation: RF MEMS series switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resulting in excellent isolation at 0.1-40 GHz.        Very Low Insertion Loss: RF MEMS series and shunt switches have an insertion loss of −0.1 dB up to 40 GHz.        Intermodulation Products: MEMS switches are very linear devices and, therefore, result in very low intermodulation products. Their performance is around 30 dB better than p-i-n or FET switches.        Very Low Cost: RF MEMS switches are fabricated using surface (or bulk) micromachining techniques and can be built on quartz, Pyrex; low temperature cofired ceramic (LTCC), mechanical-grade high-resistivity silicon, or GaAs substrates.        MEMS switched can be categorised as follows:                    RF circuit configuration—series or parallel.            Mechanical structure—Cantilever or Air-bridge.            Form of Contact—Capacitive (metal-insulator-metal) Resistive (metal-metal).                        
FIGS. 1 and 2 show a typical MEMS capacitive switch 63 which consists of a thin metallic bridge 65 suspended over the transmission line 67 covered by dielectric film 69. The MEMS capacitive switch can be integrated in a coplanar waveguide (CPW) or in a Microstrip topology. Conventional capacitive switches have a layer of dielectric between the two metal layers (bridge and t-line).
In a CPW configuration, the anchors of the MEMS switch are connected to the CPW ground planes. As seen in FIG. 2, when a DC voltage is applied between the MEMS bridge and the microwave line there is an electrostatic (or other) force that causes the MEMS Bridge to deform on the dielectric layer, increasing the bridge capacitance by a factor of 30-100. This capacitance connects the t-line to the ground and acts a short circuit at microwave frequencies, resulting in a reflective switch. When the bias voltage is removed, the MEMS switch returns to its original position due to the restoring spring forces of the bridge.
RF MEMS switches are used in reconfigurable networks, antennas and subsystems because they have very low insertion loss and high Q up to 120 GHz. In addition, they can be integrated on low dielectric-constant substrates used in high performance tuneable filters, high efficiency antennas, and low loss matching networks.
RF MEMS switches offer very low loss switching and can be controlled using 10- to 120 kΩ resistive lines. This means that the bias network for RF MEMS switches will not interfere and degrade antenna radiation patterns. The Bias network will not consume any power and this is important for large antenna arrays.
The underlying mechanism is a compact MEMS cantilever switch that is arrayed in two dimensions. The switches within the array can be individually actuated. Addressability of the individual micro switches in the array provides the means to modify the circuit trace and therefore allows fine tuning or complete reconfiguration of the circuit element behaviour.
The typical MEMS switches require typical pull down voltages of 50-100V (these can be significantly lower or higher depending on the exact configuration and material system). This is a large range to cover using a software controlled DC MEMS Switch.
The University of California, Irvine has proposed the use of a pixel antenna concept having an array of individual antenna elements that can be connected via MEMS Switches. Frequency reconfigurability is achieved by simply changing the size of the Antenna. By selecting 25 pixels an upper operating frequency of 6.4 GHz is obtained, whereas a lower frequency of 4.1 GHz is obtained by selection of all 64 pixels.
It is an object of the present invention to provide an improved reconfigurable MEMS antenna.